KINETIC THEORIES OF GRAVITATION. 



By William B. Taylor, of Washington. 



"Xon fingendum, aut excogitandum, sed inveniendum quid Nature faciat aut ferat." — Bacon. 

 "Causas rerum naturalium non plures admitti debero, quam qiue et vcrce sint, et earuru phnenome- 

 nis esplicandis svffwiant."— Newton. 



Ever since the grand demonstration by Newton in 1G82, that the moon 

 is a falling body, observing precisely the same law of decline from a 

 rectilinear path as the cannon-ball, and that it is therefore under the 

 dominion of the same force, an eager and unceasing desire has been 

 manifested to discover an antecedent or origin of this universal tendency 

 of matter. 



Even before this date, or in 1G71, the ingenious Dr. Robert Hooke 

 had endeavored to trace the cause of gravitative fall to the external ac- 

 tion of waves in a surrounding medium. He appears to have been led 

 to this reflection by observing that small bodies floating on the surface 

 of agitated water collected toward the center of disturbance or the ori- 

 gin of the waves.* 



Newton himself, as is well known, speculated on this subject, and 

 some years before arriving at his great generalization, he threw out a 

 suggestion as to the cause of terrestrial gravity in a letter to Mr. Boyle. 

 As connected with this speculation, it may be well to recur to Newton's 

 still earlier statement of his conceptions in regard to the nature and 

 action of the cether. In a letter to Mr. Henry Oldenburg, secretary of 

 the Eoyal Society of London, in January, 1G75-G, he thus unfolds the 

 hypothesis: 



"First, it is to be supposed therein that there is an retherial medium, 

 much of the same constitution with air, but far rarer, subtiler, and more 

 strongly elastic. But it is not to be supposed that this medium is of 

 one uniform matter, but composed partly of the main phlegmatic body 

 of the sether, partly of other various authorial spirits, much after the 

 manner that air is compounded of the phlegmatic body of air intermixed 

 with various vapors and exhalations; for the electric and magnetic 

 effluvia and the gravitating principle seem to argue such variety. Per- 

 haps the whole frame of nature may be nothing but various contextures 

 of some certain setherial spirits or vapors, condensed as it were by pre- 



* Posthumous Works of Eobert Hooke, edited by Richard Waller. London, 1705, pp. 

 xiv, and 184. 



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